Among fuel cells for generating electrical power utilizing an electrochemical reaction between hydrogen and oxygen, polymer electrolyte fuel cells are commonly known. The polymer electrolyte fuel cell includes a stack which is constituted from a plurality of stacked cells. The cells constituting the stack each include an anode (fuel electrode) and a cathode (air electrode), and a solid polymer electrolyte membrane having a sulfonic acid group as an ion exchange group is interposed between each anode and cathode.
A fuel gas containing a fuel gas (hydrogen-enriched reformed hydrogen obtained by reforming hydrogen gas or hydrocarbon) is supplied to the anode, while an oxygen-containing gas (oxidant gas), e.g., air, is supplied to the cathode as an oxidant. Upon the supply of the fuel gas to the anode, hydrogen contained in the fuel gas reacts with catalyst in a catalyst layer which constitutes the anode, thereby generating hydrogen ions. The generated hydrogen ions pass through the solid polymer electrolyte membrane and electrically react with oxygen in the cathode. Electrical power is thus generated through the electrochemical reaction.
Meanwhile, in fuel cell systems, in an attempt to start a fuel cell system at a low temperature, if water from when the system last stopped still remains in a fuel cell, the remaining water freezes and may cause the system to be unable to start. Even if the system can be started, product water resulting from its own reaction may freeze and cause power generation to stop.
In light of such circumstances, the power generation efficiency of fuel cells has been controlled in order to control self-heating power. In order to increase the self-heating power, a fuel cell is operated with a short supply of a reaction gas by, for example, reducing the supply of the reaction gas or causing a short-circuit between the electrodes of the fuel cell, so that an overvoltage between the electrodes of the fuel cell is increased. In this instance, when the supply of the reaction gas is reduced to maintain the voltage of the fuel cell at around 0 volts, there is a possibility that a reverse potential is generated in a cell, which causes hydrogen to be transferred to an oxygen electrode of the fuel cell and to be exhausted from an air exhaust path. Accordingly, it has been proposed that a bypass path for connecting the downstream of an air blower in an air supply path and the air exhaust path to each other, so that the external air supplied from the air blower is introduced in the air exhaust path via the bypass path to dilute the hydrogen inside the air exhaust path (see Patent Document 1).
Patent Document 1: Japanese laid-open patent publication No. 2006-73501